The present invention relates to light shielding and, more particularly, to a structure and method for shielding integrated circuit elements from light. A major objective of the present invention is to provide shielding which selectively admits light for activating predetermined circuit elements in an integrated circuit while prohibiting light from disturbing other elements in the integrated circuit.
Hybrid devices are available which take advantage of strengths of both electrical and optical technologies. For example, optical couplers can be used to couple two electrical circuits which operate at very different voltages without the danger of a device breakdown due to large potential differences. Serendipitously, semiconductors like silicon, which provide the most sophisticated of electronic circuits, are also optically active. Thus, for example, a single integrated circuit can include elements acting as photo-detectors and other elements dedicated to electronically processing photo-detections to provide electrical outputs. A single integrated circuit can be used to develop a control signal based on a pattern detected by integrated optical detectors.
The ability of semiconductor elements to respond to both optical and electrical inputs is problematic in that the operation of circuit elements intended to respond only to electrical inputs can be affected by incident light. In circuits designed for electrical inputs, outputs and processing only, this problem can be addressed by enclosing an integrated circuit in a light-tight package. However, hybrid devices must provide light access to photo-detector elements while excluding light from other circuit elements. Standard integrated circuit packages are not well-adapted to allowing light to reach certain areas of an integrated circuit while excluding it from others.
Ideally, an opaque material would be patterned directly on an integrated circuit to admit and exclude light over respectively appropriate areas. Preferably, the material would be opaque over a broad frequency range, including both infrared and visible light wavelengths. As a minimum requirement, the material should be opaque relative to the wavelengths to be detected by the photo-detector elements. Other wavelengths could be excluded by packaging or other means.
Carbon black is a pre-eminent light absorbing material, absorbing a high percentage of incident light over a broad frequency range. It can be obtained in highly pure form and can be applied over a semiconductor surface by a number of different methods. The problem with carbon black is that it is conductive and can short or otherwise interfere with circuit functions, even when applied over a conventional insulating passivation layer.
There are non-conductive light-absorbing materials, such as magnetite, but it is difficult and expensive to obtain them with the purity required for compatibility with integrated circuits. Contaminated materials introduce impurities into the integrated circuit which can impair circuit functioning.
Furthermore, it is preferable that the method used to apply and pattern a chosen opaque coating be compatible with standard semiconductor processing techniques and equipment. Very different processing would likely be costly in terms of training and equipment. Heretofore, there has been no material compatible with the electrical, optical and processing requirements discussed above.